1. Field of the Invention
This invention relates, in general, to a method of evaluating scorch in flexible polyurethane foams and particularly concerns a method of predicting the probability that flame retarded flexible polyurethane foam will scorch on curing. The method can also be used to screen flame retardants for polyurethane foams.
2. The Prior Art
The use of flame retardants in polyurethane foams is well known. Many flame retardants, however, cause the foams to scorch during the foaming reaction. In other cases, flame retarded foams can scorch during the first few hours of storage after the foaming process. This is especially likely when the new foam buns are stacked and heat transfer from the center of the buns is inhibited. Scorch not only reduces the aesthetic quality of the foam by discoloring it, but also can cause degradation of the physical properties.
It has been very difficult in the past to assess the scorch potential of flame retardants in the laboratory. Generally the small size of the foam bun produced in the laboratory allows a rapid dissipation of heat to occur, and except in the case of the least thermally stable materials any scorching or discoloration is usually minimized or undetectable. Even foam buns prepared in 1.5' .times. 1.5' .times. 3' insulated boxes do not reproducibly show scorch.
In the literature, several small scale methods for determining scorch are discussed. U.S. Pat. No. 2,915,496 discloses the use of very high water levels to produce large exotherms. Long cure cycles at elevated temperatures in ovens are disclosed in U.S. Patent No. 3,281,379. In U.S. Pat. No. 3,281,379 measurement of the induction time required for phenol oxidation is discussed. The incorporation of additives to the foam to catalyze free radical reactions is disclosed in British Pat. No. 1,407,244. Each of these methods have inherent shortcomings such as extended oven exposure times or the need to make large quantities of foam. Above all, correlation with actual commercial production has been poor. These problems have been overcome by the present invention.